The bulbs don't matter. They use X amps of current,
and if the wiring is all kosher and the GFCI is not
defective, then that amount of current goes out
through the hot wire plugged into the GFCI and
comes back through the neutral wire. It does not
matter whether they are using more or less current
than they are specified to use, as long as the current
leaving the GFCI matches the current returning to the
GFCI.
In this case it apparently does not match, at least not
all of the time. There are two paths to ground at the breaker
box, through a neutral wire and through a ground wire.
A ground wire only carries current if there is a short
somewhere and the device with the short is grounded
someway, usually with a ground wire and 3-prong plug,
grounded metal conduit, whatever. (Current can also leak
to ground through a concrete floor, a cast iron or copper
pipe that connects to ground outside, etc.)
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